This invention relates generally to electronically-controlled telecommunication switching systems. More particularly, this invention relates to a distributed processing telex exchange in which the system processing requirements have been distributed to different levels of processor control, and in which each processor in each level of control has a redundant parallel processor as a backup unit to thereby increase system reliability.
The advent of the digital computer has enabled larger and more sophisticated telecommunication systems to be designed, such as the Frederick Electronics Corporation ELTEX II/III telex exchange (switch). These computer controlled exchanges provided faster and more reliable control of the calls between telex terminations. Prior-art systems of this type are capable of handling the calls from a large number of terminations through a combination of hardware and a lot of software. However, because of the large software overhead in the software of these prior-art systems, they are limited in the number of calls they can handle and in their ability to handle higher data transmission rates (band rates). In addition, varying types of signaling sequences are required by the terminations serviced by the exchange. These sequences are produced in these prior-art systems by fixed software routines for each termination. Since each termination requires its own customized software routines for its signaling requirements, it is very difficult to make changes in the type of signaling that a given termination may require because of the changes that must be made in both the computer software and the dedicated system hardware for that termination.
For these prior-art systems, each termination serviced by the exchange was scanned according to a predetermined sequence to determine the state of the line, such as a bid-for-service, clearing, etc. Additionally, the characters transmitted to the exchange were converted to parallel data and transmitted within the exchange as a complete character. Coupling of characters by software was reasonable in telex switches of 8000 line capacity or less, even with processors having relatively high instruction execution speeds. However, it soon became apparent that higher baud rates and larger exchange sizes would be difficult to accommodate using existing designs unless a new approach to the concept of handling both the processing requirements of the exchange and the signaling requirements for the many varieties of types of signaling that now exist, as well as providing flexibility for new types.
In order to interface with subscribers to handle calls originating in terminations connected to the telex exchange or to forward calls to the terminations, the exchange must generate and respond to signals in the termination lines. Those skilled in the art refer to these signals as the protocol signaling sequences. These sequences are produced in the signal lines between the exchange and the termination as sort of a stimulus-response technique. For example, a signal is sent from the termination to the exchange. This signal is received and causes the exchange to produce a signal back to the termination that, in turn, causes the termination to send another signal back to the exchanger, and so forth. If all signal levels and pulse durations transmitted between the termination and the exchanger occur within specific time intervals, then the exchange and the termination will be able to exchange conversation data.
It would be exceedingly difficult, if not impossible, for the prior-art telex exchanges to increase the number of terminations, and accordingly, the number of calls that it can handle, by one, continuing to have dedicated software routines for each termination to handle the signaling sequence required by that termination, and two, having a single central processor attempt to handle both the call servicing and the other processing requirements of the telex exchange. Accordingly, it would be advantageous to provide a telex exchange that incorporates the computing power of the digital processor by distributing the processing requirements of the exchange to different levels of processor control to optimize the processing requirements at the levels where it is needed. Further, to provide a distributed processing exchange in which each processor at each level of control is comprised of a redundant pair of processors both processing the same data simultaneously and both processors passing the same data to every other processor in each redundant pair in each level of control to which it is connected to thereby improve the exchange's reliability. It would also be advantageous to provide a single set of hardware and software within the distributed processing telex exchange that would be capable of handling the signalling protocol for all of the various signaling types of terminations, as well as, to permit the exchange to service more terminations as data rates higher than in existing prior-art exchanges.